The present invention relates to radiation curable coatings for aluminum alloy bodies. More specifically, the invention relates to an improved primer composition for coatings containing radiation curable polymer precursors. The product is preferably polymer coated aluminum alloy sheet suitable for shaping into end panels for food and beverage containers. Another aspect of the invention relates to polymer coated aluminum alloy extrusions for architectural uses.
Aluminum alloy sheet coated with a polymeric composition on one or more surfaces is commonly used for shaping into end panels for food and beverage containers. The coating can be applied by processes such as reverse roll coating, gravure coating, electrocoating, spraying, powder coating, and forward roll coating. Coatings are applied to the sheet to better preserve foods and beverages stored in metal containers produced from the coated sheet, and to improve their taste characteristics. Coatings also improve the corrosion resistance, formability and appearance of the metal.
Commonly used commercial coating processes involve the use of solvent based systems that generate volatile organic compounds (VOC""s) into the air. To reduce or eliminate the generation of such volatile organics during the coating process, solids coating systems have been proposed for application to the metal surface during the coating process. Examples of solids coating systems are those known as ultraviolet (UV) or electron beam (EB) curable coatings. Such coating systems generally contain monomers and oligomers so that they can be applied to substrates in a fluid state. Some coating systems may also contain small amounts of a solvent to improve fluidity during application. The monomers and oligomers react to form 100% solids coatings.
However, UV and EB curable coatings often separate from the metal substrate during end formation operations.
Accordingly, a need exists for a polymer coated metal sheet which is produced using coatings that do not generate VOC""s after being cured and which has sufficient polymer adhesion to adhere to the metal during subsequent fabrication of the metal sheet into ends for beverage or food containers.
Thus, an objective of the present invention is to provide an improved primer composition for making aluminum alloy sheet using coatings that do not generate VOC""s during curing and which have sufficient adhesive strength to adhere to the metal surface during subsequent fabrication of the sheet into ends for beverage or food containers.
Although aluminum and aluminum alloys protect themselves against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and certain electrolytes, aluminum and its alloys may corrode very quickly. Such moisture and electrolytes may originate in acid rain water, water puddles on roadways, salted winter roads, or food materials, among others. Coatings, such as organic polymer and silicone coatings, may protect aluminum substrates to a limited extent by themselves, but are usually poorly adherent without an intermediate treatment or layer. Also, thin coatings are frequently porous, and therefore require an intermediate coating or layer to enhance corrosion resistance of the aluminum substrate.
Accordingly, there is a need to treat aluminum substrates with primers or other chemicals that provide improved corrosion resistance as well as strong bonding affinity for subsequent coatings. Suitable aluminum substrates for practice of the present invention include sheet, plate, castings, and wrought products such as forgings and stampings.
In the prior art, chemical conversion coatings have been formed on aluminum by xe2x80x9cconvertingxe2x80x9d a surface of the metal into a tightly adherent coating, part of which consists of an oxidized form of aluminum. Chemical conversion coatings provide high corrosion resistance and improved bonding affinity for polymer coatings. A chromate conversion coating is typically provided by contacting aluminum with an aqueous solution containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. In recent years, concerns have arisen regarding the pollution effects of chromates and phosphates discharged into waterways by such processes. Because of the high solubility and strongly oxidizing character of hexavalent chromium ions, expensive waste treatment procedures must be employed to reduce the hexavalent chromium ions to trivalent chromium ions for waste disposal.
Attempts have been made in the prior art to produce acceptable chromate-free conversion coatings for aluminum. For example, some chromate-free conversion coatings contain zirconium, titanium, hafnium and/or silicon, sometimes combined with flourides, surfactants and polymers such as polyacrylic acid. In spite of the extensive efforts that have been made previously, there is still no entirely satisfactory non-chromate conversion coating or primer for improving the adhesion and corrosion resistance of coated aluminum. Two of the major problems are that the chromate free conversion coatings usually provide weaker adhesion of the subsequent coating to the substrate, or the chromate free conversion coatings usually provide less corrosion resistance in aggressive environments, or both.
A principal objective of our invention is to provide aluminum with a chromium free primer layer which provides better corrosion resistance and better adhesion of subsequent coatings than a chromate conversion coating.
A related objective of our invention is to provide coated aluminum having a primer layer comprising a reaction product of a vinylphosphonic acid-acrylic acid copolymer and an aluminum oxide or hydroxide layer on the sheet.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.
In accordance with the present invention there is provided a process for making an aluminum-polymer composite. The composite comprises an aluminum alloy body coated with a radiation cured coating composition. The aluminum alloy body may be a casting, extrusion, plate, wrought product, or sheet and is preferably a sheet having a thickness of about 0.008 to 0.015 inch. Some aluminum alloys suitable for the present invention include aluminum-manganese alloys of the AA 3000 series, aluminum-magnesium alloys of the AA 5000 series, and aluminum-magnesium-silicon alloys of the AA 6000 series. For making polymer coated sheet suitable for shaping into food container bodies or food or beverage container end panels, we prefer aluminum-magnesium alloys of the AA 5000 series and particularly the AA 5042 and AA 5182 alloys. For making polymer coated sheet suitable for use on vehicle bodies we prefer aluminum-magnesium-silicon alloys of the AA 6000 series.
Aluminum alloys suitable for container end panels such as AA5182 are provided as an ingot or billet or slab by casting. Before working, the ingot or billet is subjected to elevated temperature homogenization. The alloy stock is then hot rolled to provide an intermediate gauge sheet. For example, the material may be hot rolled at a metal entry temperature of about 700-950xc2x0 F. to provide an intermediate product having a thickness of about 0.130 inch to about 0.190 inch. This material is cold rolled to provide a sheet ranging in thickness from about 0.007 to 0.014 inch. A preferred metal sheet is AA5182 aluminum alloy sheet in either the H19 or H39 temper.
Aluminum alloys such as AA5042 are provided as an ingot that is homogenized. This procedure is followed by hot rolling to an intermediate gauge of about 0.125 inch. Typically, the intermediate gauge product is annealed, followed by cold rolling to a final gauge of about 0.007 to 0.014 inch. A preferred metal sheet is AA5042 aluminum alloy sheet in the H2E72 temper.
The aluminum alloy sheet is generally cleaned with an alkaline surface cleaner to move any residual lubricant adhering to the surface, and then rinsed with water. Cleaning may be avoided if the residual lubricant content is negligible.
The cleaned sheet surface is then primed with a primer composition comprising an aqueous solution of about 1-20 g/L of a vinyl phosphonic acid-acrylic acid copolymer VPA-AA copolymer). Solutions containing about 4-10 g/L of the copolymer are preferred. The copolymer usually comprises about 5-50 mole % vinylphosphonic acid, preferably about 20-40 mole %. A particularly preferred VPA-AA copolymer contains about 30 mole % VPA and about 70 mole % AA. The solution has a temperature of about 100-200xc2x0 F., more preferably about 120-180xc2x0 F. A particularly preferred solution has a temperature of about 170xc2x0 F.
The sheet surface may be dipped into the primer composition or the composition may be roll coated or sprayed onto the sheet surface. A preferred continuous cleaning and pretreating line is operated at about 1000-1500 feet per minute. A contact time of about 6 seconds between the sheet surface and the primer composition is sufficient when the line is operated at 1000 feet per minute. The VPA-AA copolymer reacts with the oxide or hydroxide coating to form a primer layer on the sheet surface.
Optionally, the primed sheet may be rinsed with water to remove a portion of the VPA-AA copolymer unreacted with the oxide or hydroxide coating. The rinse water preferably has a temperature of about 170-180xc2x0 C. The rinse water is concentrated by removing excess water so that the VPA-AA copolymer can be recycled. Some preferred concentrating techniques include reverse osmosis and membrane filtration.
The polymer is applied to the primer layer on the aluminum alloy sheet as a radiation curable polymer precursor. Radiation curable polymer precursors are monomeric and/or oligimeric materials such as acrylics, methacrylates, epoxies, polyesters, polyols, glycols, silicones, urethanes, vinyl ethers and combinations thereof which have been modified to include functional groups and optionally photoinitiators that trigger polymerization upon the application of ultraviolet (UV) or electron beam (EB) radiant energy. Such polymer precursors include acrylated aliphatic oligomers, acrylated aromatic oligomers, acrylated epoxy monomers, acrylated epoxy oligomers, aliphatic epoxy acrylates, aliphatic urethane acrylates, aliphatic urethane methacrylates, allyl methacrylate, amine-modified oligoether acrylates, amine-modified polyether acrylates, aromatic acid acrylate, aromatic epoxy acrylates, aromatic urethane methacrylates, butylene glycol acrylate, silanes, silicones, stearyl acrylate, cycloaliphatic epoxides, cyclohexyl methacrylate, ethylene glycol dimethacrylate, epoxy methacrylates, epoxy soy bean acrylates, glycidyl methacrylate, hexanediol dimethacrylate, isodecyl acrylate, isoctyl acrylate, oligoether acrylates, polybutadiene diacrylate, polyester acrylate monomers, polyester acrylate oligomers, polyethylene glycol dimethacrylate, stearyl methacylate, triethylene glycol diacetate, and vinyl ethers.
The polymer coatings for use in this invention are thermosetting polymers that cross link and cure when exposed to suitable radiation sources.
The preferred polymer precursors are acrylated epoxy monomers and oligomers polyester acrylates, and silicone monomers.
Photoinitiators suitable for use in this invention are materials which absorb UV and EB radiant energy and form reactive free radicals, cations, or anions which initiate polymerization of monomeric and oligomeric materials. Such materials include acryloins, ketones, substituted benzoquinones, substituted polynuclear quinones, halogenated aliphatic, alicyclic and aromatic hydrocarbons, and mixtures thereof. In an embodiment of our invention illustrated in the Examples, UV radiation interacts with a cationic photoinitiator to addition polymerize an epoxy precursor.
Photoinitiators may not be necessary for use with polymeric precursors that contain function groups that are sufficiently reactive to polymerize upon irradiation particularly with EB radiation.
Preferred polymer resins precursor systems are sold commercially by Sun Chemical Company under the registered trademarks Sunbeam and Suncure.
The polymer coating composition may also optionally contain additives such as dyes, pigment particles, anticorrosion agents, antioxidants, adhesion promoters, light stabilizers, lubricants, and mixtures thereof.
The polymer precursor coating composition may be applied to the sheet by any of several techniques, including gravure coating, slot coating, forward roll coating, reverse roll coating, spraying, powder coating, and electrostatic coating. Reverse roll coating is particularly preferred. The polymer precursor coating is preferably applied onto the metal sheet as a single layer. Preferably, both sides of an aluminum alloy sheet are coated with a polymer precursor coating leaving a thickness of about 0.01-0.5 mils (1-13 microns).
To cure the polymer precursor coating composition, the aluminum-polymer composite sheet is irradiated with an electron beam. The electron beam polymerizes and cross links the coating. The radiation dose is about 2-20 megarads, preferably about 5-15 megarads. Electron beam radiation is the preferred energy for curing the polymer precursor coatings used for this invention, however, alternatively, UV radiation may also be used. The radiation curing step does not result in any substantial temperature increase in the coating. The coating temperature is less than about 250xc2x0 F. (121xc2x0 C.) after curing.
Optionally the composite sheet may then be heated by convection or induction heating to temperatures between 350-450xc2x0 F. The exposure time needed for this heating step will depend upon the thickness of the substrate being coated, and the speed at which the substrate (e.g., coil stock) traverses through the convection or induction heating units. A typical exposure time may range from 10-15 seconds.
The inventors believe the polymer precursor coating applied to the metal sheet is sufficiently viscous to bridge peaks in the irregular microsurface of the metal sheet. When the precursor coating is polymerized by radiant energy, the polymer networks across the metal surface peaks. By networking across the peaks, the polymer does not adhere as strongly to the metal as if the polymer inhabited the pockets, grooves, and other features of the metal microsurface. Heating of the metal-polymer composite sheet causes the polymer to flow into the metal surface irregularities. Upon cooling of the sheet, the polymer hardens within the irregularities in the metal surface thereby increasing the strength of the polymer adherence to the metal sheet.
The irradiated and heat treated composites sheets are shaped into container end panels for food and beverage containers. Easy open end panels for carbonated beverages are generally shaped by stamping metal blanks between shaping dies.
In a preferred embodiment of the invention, sheets of metal coated with a polymer precursor are exposed to EB radiation and heated before the sheets are formed into container ends. However, heating of a metal sheet coated with a polymer precursor prior to irradiation with EB energy is also within the scope of this invention. Also, sheets of metal coated with a polymer precursor can be exposed to EB radiation, formed into container ends and heated within the scope of this invention.
The invention will now be further described with reference to a number of specific examples which are to be regarded solely as illustrative and not as restricting the scope of the present invention.
In accordance with the present invention there is provided an AA5182 aluminum alloy sheet in the H19 temper having a sheet thickness of about 0.0080-0.0090 in (0.20-0.23 mm) and preferably about 0.0084 in (0.21 mm).
The natural oxide coating on aluminum surfaces is generally sufficient for practice of the invention. The natural oxide coating ordinarily has a thickness of approximately 30-50 angstroms. Also, the oxide coating resulting after cleaning, cleaning and etching, or cleaning, etching, and desmutting the aluminum is sufficient for practice of our invention. It is not necessary to grow the oxide coating by treatments such as anodic oxidation or hydrothermal treatment in water, water vapor or aqueous solutions, although this is possible.
The aluminum to be coated is cleaned with alkaline or acid surface cleaner to remove any residual organic contaminants adhering to the surface, and then rinsed with water. Cleaning might be avoided if the organic contamination is negligible.
The cleaned aluminum is then primed with a primer composition comprising an aqueous solution of about 0.1-200 g/L of a vinyl phosohonic acid-acrylic acid copolymer (VPA-AA copolymer). Solutions containing about 1-20 g/L of the copolymer are preferred. The copolymer usually comprises about 5-50 mole % vinylphosphonic acid, preferably about 20-40 mole %. A particularly preferred VPA-AA copolymer contains about 30 mole % VPA and about 70 mole % AA. The solution has a temperature of between 50xc2x0 F. and 200xc2x0 F. There is an inverse relationship between time and temperature required: the lower the temperature, the longer the time required, or the shorter the time available, the higher the temperature required. A xe2x80x9cRoom Temperaturexe2x80x9d of 65xc2x0 F. to 85xc2x0 F. is preferred with an immersion time of 15 seconds to 2 minutes, which minimizes equipment and heating requirements. Where speed is critical, a temperature of 170xc2x0 F. and a time of 15 seconds is preferred. Many different combinations of time and temperature are possible.
The aluminum surface may be dipped into the primer composition or the composition may be roll coated or sprayed onto the sheet surface. The VPA-AA copolymers reacts with the oxide or hydroxide coating to form a primer layer on the sheet surface.
Optionally, the aluminum may be rinsed with water to remove a portion of the VPA-AA copolymer unreacted with the oxide or hydroxide coating. The rinse water may have temperature of 35xc2x0 F. to 200xc2x0 F. Room temperature water is preferred, unless quick drying is required, in which case heated water speeds the water drying process. The rinse water may be concentrated by removing excess water so that the VPA-AA copolymer can be recycled. This improves the economics of the process, but does not affect the invention. Some preferred concentrating techniques include reverse osmosis and membrane filtration.
The roll coating apparatus applies to the aluminum sheet a coating of a polymer precursor having a thickness of approximately 0.0001-0.0005 in (2.5-13 microns), thereby to form an aluminum-polymer composite.
The polymer coating in the composite is irradiated by an electron beam. A suitable electron beam generator is commercially available from Energy Sciences, Inc. of Wilmington Mass. under the trade designation ESI xe2x80x9cELECTROCURExe2x80x9d EB SYSTEM.
Optionally, the surface coated with the UV or EB coatings may then be subjected to thermal or induction heating by raising the metal temperature to a range of 350-450xc2x0 F.
Finally, the aluminum-polymer composite is cooled (preferably with water) and dried.